You need IOS which support Autonomus AP mode:
k9w7 - autonomous (or "site survey") IOS k9w8 - full lightweight IOS rcvk9w8 - lightweight recovery image
So any image with k9w7, for example c1240-k9w8-mx.124-25e.JAP4. If it’s k9w8, you need controller for the configuration.
Step 1: Delete existing lightweight IOS
delete /recursive /force flash:c1240-k9w8-mx.124-25e.JAP4
Step 2: Power off the AP, press mode button for a while. The AP will fallback recovery mode with IP address in 10.0.0.0/24 range.
Step 3: Connect the AP LAN with you laptop/PC and set IP 10.0.0.2/24. Run tftp server.
Step 4: Type the following command to initialize the and upload new IOS.
ap: tftp_init ap: ether_init ap: flash_init ap: tar -xtract tftp://10.0.0.2/c1240-k9w7-tar.124-25d.JA2.tar flash: ap: set BOOT=flash:/c1240-k9w7-tar.124-25d.JA2 ap: reset
I am using WPA for security, bellow link for full configuration:
I found following video very help, specially if want to know more about the recovery process.